In some power semiconductor switches, high voltages can be applied and high currents can be carried. Short circuits or excessively increased currents can therefore rapidly lead to the thermal destruction of the power semiconductor switches. Therefore, power semiconductor switches can have protective circuits for detecting a short-circuit or overcurrent state. One possibility for detecting these states is the indirect monitoring of the current through the power semiconductor switch on the basis of a voltage dropped across the power semiconductor switch. After the power semiconductor switch has been switched on, said voltage should fall quickly from a relatively high level in the switched-off state (the “switched-off state” or “OFF state” is a state of the power semiconductor switch in which the latter is “open” and carries no current) to a relatively low level in the switched-on state (the “switched-on state” or “ON state” is a state of the power semiconductor switch in which the latter is “closed” and can carry current). Correspondingly, a control signal of a power semiconductor switch (for example a gate-emitter driver signal) has an ON state, in which it keeps the power semiconductor switch closed, and an OFF state, in which it keeps the power semiconductor switch open.
The solid curve 678 at the bottom left in FIG. 6 shows an exemplary profile of a collector-emitter voltage of an IGBT (an IGBT is an exemplary power semiconductor switch) during the switchover process from a switched-off state into a switched-on state (the profile of an associated exemplary control signal 630 is illustrated at the top left). As illustrated, the collector-emitter voltage falls sharply to a very low value (close to 0 volts). An exemplary short-circuit behavior of an IGBT is illustrated at the bottom right in FIG. 6 (solid curve 678). In contrast to normal operation, the collector-emitter voltage does not fall to the very low value; simultaneously, however, high currents can flow in the IGBT (for example between three times and ten times the nominal current of the IGBT). In other short-circuit cases, the collector-emitter voltage indeed firstly falls to the value in normal operation, but then rises again. This results in a high thermal loading of the power semiconductor switch, which can incur damage after a relatively short time. In this regard, for example, some IGBTs in the switched-on state withstand a short circuit for approximately 10 μs without incurring damage. Therefore, the protective circuits for detecting a short-circuit or overcurrent state in this time range can ensure that the power semiconductor switch is switched off. Similar characteristics can also be found in other power semiconductor switches besides IGBTs. An overcurrent state, like a short-circuit state, can be manifested by an increased collector-emitter voltage. However, in the overcurrent case, the collector-emitter voltage can be closer to a collector-emitter voltage in the normal case in comparison with the short-circuit case.
The outlined differences in the profile of the collector-emitter voltage between normal operation and the short-circuit and/or overcurrent case can be utilized in protective circuits for detecting a short-circuit or overcurrent state, in order to detect a short-circuit or overcurrent state. It is thus possible to define a threshold value for the collector-emitter voltage, which threshold value is used to detect the presence of a short-circuit or overcurrent state. By way of example, if the collector-emitter voltage rises above the threshold value (see FIG. 6, bottom right), a short-circuit or overcurrent state can be detected.